1. Technical Field
This invention relates generally to rechargeable electric batteries particularly suited for applications, e.g., implanted medical devices, where a battery cannot be easily replaced. More particularly, the invention relates to rechargeable lithium batteries configured to tolerate deep discharging to zero volts without permanently damaging the battery's energy storing capability.
2. Background Art
Rechargeable electric batteries are employed in a wide range of applications, e.g., consumer products, medical devices, and aerospace/military systems, which respectively impose different performance requirements. In some applications, e.g., implanted medical devices, it is important that the battery be able to reliably maintain its performance characteristics over a long useful life despite extended periods of inactivity. Implanted medical device applications impose special requirements on a battery because the medical device needs to be highly reliable to perform critical tasks, the battery may remain inactive and uncharged for extended periods, e.g., several months, and it is difficult and/or expensive to replace a battery. Analogous conditions exist in various aerospace/military applications. For example, a rechargeable battery may be deployed to power a satellite in deep space where it cannot be replaced and must be able to operate over a long life under varying conditions, including long periods of inactivity. Military applications often demand similar performance specifications since military hardware can be unused for several months but must remain ready to be activated. Current battery technology requires stored batteries to be charged every few months to avoid a permanent reduction in energy storing capability.
In order to avoid unnecessary surgery to replace a damaged battery in an implanted medical device, it is desirable that a battery perform reliably over a very long life, i.e., several years, under a variety of conditions. Such conditions can include extended periods of non-use which may allow the battery to deeply self discharge to zero volts. It is typical for prior art rechargeable lithium batteries to suffer a permanent capacity loss after discharging below 2.5 volts. To avoid such capacity loss, it is important to regularly charge prior art lithium batteries.
Existing rechargeable lithium batteries typically consist of a case containing a positive electrode and a negative electrode spaced by a separator, an electrolyte, and feedthrough pins respectively connected to the electrodes and extending externally of the case. Each electrode is typically formed of a metal substrate that is coated with a mixture of an active material, a binder, a solvent, and sometimes a conductor. In a typical battery design, the electrodes comprise sheets which are rolled together, separated by separator sheets, and then placed in a prismatic or cylindrical case. Positive and/or negative feedthrough pins (i.e., terminals) are then connected to the respective electrodes and the case is filled with electrolyte and then sealed. The negative electrode is typically formed of a copper substrate carrying graphite as the active material. The positive electrode is typically formed of an aluminum substrate carrying lithium cobalt dioxide as the active material. The electrolyte is most commonly a 1:1 mixture of EC:DEC in a 1.0 M salt of LiPF6. The separator is frequently a microporous membrane made of a polyolefin, such as a combination of polyethylene and/or polypropylene which can, for example, be approximately 25 microns thick.
Batteries used in implanted medical devices can be charged from an external power source utilizing a primary coil to transfer power through a patient's skin to a secondary coil associated with the implanted medical device. The secondary coil and an associated charging circuit provide a charging current to the battery. Protection circuitry is typically used in conjunction with prior art lithium batteries to avoid the potential deleterious effects of over charging or overdischarging the battery. Such protection circuitry can terminate charging if the voltage or temperature of the battery exceeds a certain level. Moreover, it is common to also incorporate low voltage protection to disconnect the battery from its load if the voltage of the battery falls below a certain lower level. This latter precaution is taken to prevent permanent damage to the battery that will likely occur if the voltage on an electrode exceeds a Damage Potential Threshold (DPT). For example, it is well known in the industry that discharging a lithium battery to below 2.5 volts and storing it for an extended period of time will likely result in a permanent loss of battery capacity. Despite incorporating low voltage cutoff protection to disconnect the battery from its load if the voltage falls below a certain threshold, typical prior art batteries will slowly self-discharge further causing the voltage of an electrode to exceed the Damage Potential Threshold.